The present invention pertains generally to illumination optics, and more particularly systems and methods for illumination of objects in machine-vision systems.
During the manufacture of certain products, such as electrical components, it is necessary to be able to provide high-intensity illumination so that components can be thoroughly inspected with a machine-vision system. Often, the light source needed includes one or more light sources, for example a ring-shaped flashtube or a number of light-emitting diodes arranged along a circle or a remote light source that drives light into a number of optical fibers arranged along a circle, surrounding the lens of a video camera such that the object being imaged by the video camera is illuminated with light angled in towards the optical axis of the camera from the light source surrounding the lens. It is desirable that the light source or sources are arranged such that no light shines directly from the light sources into the lens.
Typically, a xenon flashtube or laser-based single-point source or other high-intensity light source is used for providing light into fiber-optic-based ring source. Such systems, however, are costly, very, large and bulky, and can interfere with the placement of other components in the machine-vision system. This is particularly troublesome when the components being measured or inspected are extremely small. Xenon flashtube light sources also tend to exhibit up to about a five percent (5%) flash-to-flash variation in intensity which makes accurate measurements of certain characteristics difficult. Single-point source systems are also generally limited to emitting light radially from only one single point, which is of limited value when shadows are problematic, such as, when inspecting a grid of electrical connectors. Specifically, light from only one or just a few point sources only illuminates the first over-sized or over-height electrical connector and, due to shadows from the first object encountered, does not provide proper illumination which would determine if other objects behind this particular first object are missing, of the incorrect size or height, or perhaps in the wrong position.
Conventional illumination systems produce a light which can be too bright in certain areas and too dim in other areas. Often, the end-result is xe2x80x9cbloomxe2x80x9d, especially when viewing white, lightly colored, or very reflective objects which are near other objects which need to be viewed by a machine-vision camera. In order to get enough light on the other objects which need to be viewed, the aperture on the camera cannot be xe2x80x9cstopped downxe2x80x9d in order to prevent overexposure of the bright objects. Specifically, the area is illuminated to such an extent that the entire image appears to be the same bright saturated white color (or, if a monochromatic light source is used, saturated at whatever color is used) as viewed by the machine-vision camera and system. Such extreme brightness also poses a danger of blinding, at least temporarily, human workers nearby.
Quite often, illumination sources either leave certain portions of the scene in shadows, or provide too much light in certain areas, while leaving other areas with too little light. In other cases, the illumination source is too bulky and gets in the way of other components of the machine-vision system, associated robots, manipulators, and/or human workers.
The optimal light-source-to-optical-axis angle can vary depending on the object being inspected. One shortcoming of conventional ring light sources is the cost and difficulty in changing the angle between the light sources relative to the optical axis, and in changing the spread and/or focus of the light from ring-light source.
Thus, what is needed is an ring-light illumination system and method which is compact, provides control over both the angle between the light source and the optical axis of the camera, as well as the spread and/or focus of the light from ring light source, so that even extremely small parts can be quickly and adequately inspected and accurately viewed or measured with a machine-vision system. Another need is to provide a compact illumination source, preferably monochromatic, which can be focused to provide uniform multi-directional light onto objects from all sides while avoiding light going directly from the light sources to the lens of the camera. Another need is to provide a compact monochromatic LED (light-emitting diode) illumination source, which can be changeably focused to provide uniform multi-directional light onto objects. Another need is to have such an LED illumination source be pulsed with a relatively high-power, low duty-cycle power source.
The present invention takes, advantage of the efficiency of high-brightness red, infra-red, blue, white, or other color LEDs arranged in one or more circular rows, and the properties inherent to a reflective focusing element such as a turned angled reflective ring to produce an illumination source for machine-vision systems. The illumination source exhibits multidirectional ring-illumination properties which are useful for illumination of small components (which are being inspected or measured) without unwanted shadows. One embodiment provides a darkfield illumination system. One embodiment of the present invention uses a strobed (or pulsed) power supply to drive the LEDs. Yet another embodiment of the present invention uses a xenon strobe ring-light source and a backplane slit in place of the row of LEDs 25. In one such xenon strobe embodiment, a color filter is also placed in series with the light path in order to obtain a monochromatic light. While xenon flashtube light sources tend to exhibit a five percent (5%) flash-to-flash variation in intensity which makes accurate measurements of certain characteristics difficult, they are useful in certain cases where intense white, or especially ultraviolet, light is desired.
The present invention provides a compact ring-light generator which has little, if any, shadowing. The present invention also provides an inexpensive method for changing the light-source-to-optical-axis angle. The present invention also provides an inexpensive method for changing the spread and/or focus of the light from ring light source.
The present invention provides a method and apparatus which provide an illumination source for illuminating an object in a machine-vision system having a machine-vision camera, the camera having an optical axis. One embodiment of the illumination source includes a ring-light source emitting light from a plurality of points, the points being along one or more circles, a focusing element, the focusing element including an angled ring reflector to direct rays from the ring light source at an angle generally towards the optical axis. One embodiment provides a replaceable ring reflector for changing the light-source-to-optical-axis and/or changing the spread and/or focus of the light from ring light source. One embodiment provides light from multiple directions in order to reduce shadowing. One embodiment provides light to illuminate the inside of, for example, an aluminum beverage can before it is filled and sealed.
One embodiment uses a ring-reflector focusing element which includes a first conical section reflective surface at a first conical angle to the optical axis. Another embodiment further includes a second conical section reflective surface at a second conical angle to the optical axis.
One embodiment includes an illumination source for illuminating an object in a machine-vision system, the system having an optical axis. The illumination source includes a ring light source and a ring reflector. The ring light source emits light from a plurality of points or from a line, the points or line being substantially in a plane that intersects the optical axis, each of the points or the line disposed at least a first distance from the optical axis and less than a second distance from the optical axis. The ring reflector has an exit opening through which the optical axis passes, the emitted light from the ring light source being generally directed centered on lines that intersect a reflecting surface of the ring reflector, the ring reflector reflecting the emitted light from the light source through the exit opening inwards and generally toward the optical axis or an area around the optical axis.
In one such embodiment, the ring light source includes a plurality of light-emitting diodes (LEDs) arranged substantially along a circle disposed perpendicular to and centered on the optical axis. In another such embodiment, each LED has an focal centerline emission axis along which emission is centered, and each LED""s emission axis is parallel to the optical axis. In yet another such embodiment, each LED has an focal centerline emission axis along which emission is centered, and each LED""s emission axis is perpendicular to the optical axis.
In one embodiment, the light emitted from the LEDs is two or more selected from the following: infra-red, red, amber, yellow, green, blue, violet, ultraviolet, or white in color. In another such embodiment, the light emitted from the LEDs is primarily within an angle of about 5xc2x0 from a focal centerline of each individual LED.
One embodiment further includes a focusing element that includes a cylindrical ring lens having at least one convex face.
In one embodiment, the ring light source includes a ring-shaped flashtube located substantially along a circle disposed perpendicular to and centered on the optical axis, and further includes an enclosure having a slit located between the ring-shaped flashtube and the ring reflector, wherein the slit allows light from the flashtube to fall on a reflecting surface of the ring reflector.
In one embodiment, the ring reflector has a surface that enhances its reflectivity at one or more selected wavelengths of the ring light source.
In one embodiment, the ring reflector and ring light source are configured to produce a darkfield illumination.
Another aspect of the present invention is a method for illuminating an object located along anoptical axis. The method includes the steps of (a) emitting light from a plurality of points or from a line, the points or line being substantially in a plane that intersects the optical axis, each of the points or the line disposed at least a first distance from the optical axis and less than a second distance from the optical axis; and (b) reflecting the emitted light from the light source inwards and generally towards the object at the optical axis or in an area around the optical axis.
In one such embodiment, the object is an electrical connector, and the method further includes the step of acquiring a machine-vision image of the electrical connector. One application for such a method is inspecting ball-grid arrays.
Another aspect of the present invention provides a machine-vision illumination system. The system includes an imaging device, an image processor coupled to the imaging device, and an illumination source coupled to the image processor. The illumination source includes a ring light source and a ring reflector. Other aspects of such a system are described above.
Another aspect of the present invention provides a ring reflector for use in reflecting light from a ring light source The ring reflector includes a substantially circular exit opening though which the optical axis passes and a reflective surface surrounding the exit opening. The reflective surface extends from approximately a first circle having a first radius, to approximately a second circle and having a second radius, the second radius being larger than the first radius, the first radius being larger than the difference between the second radius and the first radius.
In one such embodiment, the reflective surface includes a conical section. In another such embodiment, the reflective surface includes a plurality of adjoining conical sections. In yet another such embodiment, the reflective surface includes a circularly-rotated parabolic section. In still another such embodiment, the reflective surface includes a plurality of reflective facets. In one such embodiment, for a plurality of the facets, a line normal to the facet surface passes through the optical axis. Another aspect of the present invention provides a reflective surface that is configurable to change the angle at which it reflects light.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings.